1. Field of the Art
This invention relates to a process for production of cellular concrete by the so-called pre-foam technique by using a slurry of a rapid-hardening cement compound. More particularly, the invention relates to a process for producing the cellular concrete characterized by the means for controlling the setting of the foamed or aerated slurry of the rapid-hardening cement compound.
Cellular concrete has been the focus of much attention because it affords reductions in weight and cost in concrete structures. Prefabrication of concrete products has been developed, and a variety of cellular concrete panels have come into use.
Such cellular concrete is represented by autoclaved light-weight concrete (hereinafter referred to as ALC). ALC has a large market for use in stable building materials and is essentially light-weight concrete comprised of crystalline calcium silicate hydrate of tobermorite species which has been produced by aging set prefoamed products in an autoclave under pressurized hydrothermal conditions.
The ALC now produced on an industrial scale can be classified into the so-called post-foam type and pre-foam type. The former post-foam process is carried out by introducing a hydraulic cement compound slurry into a mold and foaming it with hydrogen gas generated from the reaction of aluminum powder added into the cement compound with an alkali component such as cement or lime. According to this process, the height of the foam produced in a mold is limited to about 60 cm in order to secure uniformity of foaming conditions of upright direction in view of the static pressure of the cement compound slurry under foaming in the mold. Thus, in order to increase the efficiency in the use of costly molds, panels or slabs (before curing) of 60-cm width due to the above mentioned height of the foamed product are produced by slicing the resulting foamed products vertically to a predetermined thickness.
As is clear from the above described procedure, the conventional cellular concrete panels of the post-foam type are not satisfactory with respect to large-sized products, complicated shapes and designs, etc.
Such problems can be substantially solved by the so-called pre-foam process wherein foam is introduced into a hydraulic cement compound slurry and then the foamed slurry is poured into a mold. In this case, however, one mold is used for each panel or slab. Thus, it is essential to shorten the time between pouring and demolding by causing rapid setting of the cement compound slurry in order to increase the efficiency in use of the costly mold. On the other hand, it is necessary (i) to prevent such phenomena as local setting of the cement slurry in the mold in the procedure between pouring and demolding, segregation of high-density components, and defoaming, and (ii) to carry out surface treatment such as scraping or leveling of protuberant portions over the mold, planing of the surface, or stamping design patterns. Thus, it is also essential that the cement slurry under setting have suitable consistency for a period necessary for such surface treatment.
2. Prior Art
From this point of view, a possible measure which may be considered is the incorporation of a setting retardant or retarder into the rapid-hardening cement compound slurry.
A variety of the setting retarders have been proposed for such cement compound slurry. The conventional retarders, however, are not suitable for the above mentioned purpose, because the patterns of their setting retardation are not suitable.
More specifically, the figure in the attached drawing shows the exothermic conditions of hydraulic cement compound slurries under their hydration reactions with elapse of time. In the drawing, the vertical axis indicates the degree of setting or strength because the exotherm is generally correlated with the degree of setting. To the best of our knowledge, the conventional setting retarders exhibit the pattern of the curve C.
In the drawing, the curve B shows the pattern of a cement slurry which contains no retarder, wherein the setting proceeds progressively with elapse of time. The cement slurry shown by the curve B is advantageous with respect to the segregation of high-density components in the slurry and the escape of the introduced foam because its consistency reaches a desirable level rather rapidly but is inferior in the workability or reproducibility of the pouring and the above mentioned surface treatment because its setting passes rather rapidly over the level or region of consistency necessary for the pouring and surface treatment.
The curve C shows the case where a setting retarder is used. The setting pattern as shown by the curve C has good workability for the pouring into the mold because the setting reaction is strongly restricted for some period of time. In this restricted stage, however, the surface treatment is not possible, and also the segregation of high-density components is unavoidable because sufficient consistency is not yet obtained. After termination of the setting restriction, there are also raised the same problems as in the case of curve B.
The curve A shows a setting pattern wherein a level of consistency (A.sub.1) is exhibited and the condition of such consistency level is maintained (A.sub.1 .fwdarw.A.sub.2) for some period of time. A setting pattern wherein the period between A.sub.1 and A.sub.2 is amply long can be said to be an ideal pattern because the above mentioned surface treatment becomes possible.
To the best of our knowledge, however, there is no setting retarder which exhibits such an ideal pattern. Conventional retarders generally produce the curve B or the curve A wherein the period between A.sub.1 and A.sub.2 is very short. Conventional setting retarders exhibiting such patterns are exemplified by an alkali metal salt of citric acid or tartaric acid as well as a combination of an alkali metal carbonate and citric acid or tartaric acid.